The phenomenon of superconductivity, that is, zero electrical resistance, possessed by many metals at near absolute zero temperatures, has received steadily increasing attention in recent years due to the development of materials which exhibit this property at sufficiently high temperatures, while carrying relatively high currents in the presence of sufficiently great magnetic fields as to be of commercial utility. Among the more useful of the superconducting materials developed to date are the intermetallic compounds Nb.sub.3 Sn and V.sub.3 Ga. These materials have sufficiently good superconductive properties as to render them attractive in the development of useful electrical machinery.
Recently developed processes for the manufacture of Nb.sub.3 Sn have generally involved the so-called "bronze process" in which rods or wires of Nb are dispersed throughout a matrix consisting of a CuSn bronze. The assembly is worked to a desired final size and heat treated, at which time Nb.sub.3 Sn is formed at the interfaces between the Nb rods and the bronze matrix by diffusion of the Sn from the bronze. See, for example, U.S. Pat. No. 3,918,998. Refinements of the bronze process include providing a quantity of good electrical conductor such as pure Cu in close proximity to the Nb.sub.3 Sn filaments and isolating this pure Cu from diffusion of Sn which would destroy the high electrical conductivity of pure Cu, by interposing a layer of material impermeable to Sn therebetween such as, for example, Ta; see, e.g. U.S. Pat. No. 4,205,199. The same process is used to form multifilamentary V.sub.3 Ga; V rods are disposed in a CuGa bronze matrix.
A quantity of a good electrical conductor in close proximity to the superconductor material is useful as an alternate current path or shunt in situations where it is likely that some fraction of the superconductive filaments will return to the normally-conducting state, which can happen, for example, in a rapidly-varying magnetic field. As discussed in a Progress Report of Dec. 15, 1976 on "Experimental Evaluation of Forced Flow Cooled Superconductors" of the Francis Bitter National Magnet Laboratory at MIT, a cryostabilized superconductor must inherently include a stabilizer. In the case of a forced flow cooled conductor the stabilizer must carry the total current while the superconductor is above its critical temperature. The stabilizer must further carry any current that the superconductor cannot handle during the current-sharing phase of recovery. The bronze process may be used to achieve multifilamentary intermetallic superconductors which are stabilized by the provision of a quantity of a good electrical conductor. However, other methods which do not employ the bronze process may also be used to fabricate multifilament superconducting elements, see Marancik U.S. Pat. Nos. 4,646,428 and 4,411,712, both incorporated herein by reference. The brittle nature of the intermetallic compounds Nb.sub.3 Sn and V.sub.3 Ga and the difficulty of bending them, especially when there is a sharp curvature, have been noted in these patents. As described in Marancik et al U.S. Pat. No. 4,646,428, the centers of a plurality of copper tubes are filled with tin and drawn to form Cu-Sn wires which are cabled around a core Nb wire; a plurality of these strands are provided in a copper tube, or a copper foil or finely wound copper wire; and a plurality of said tubes are packed into a copper can to form a billet which is drawn to produce a multifilament wire; and heat treatment is applied to cause the tin to diffuse and form the intermetallic Nb.sub.3 Sn at the surface of the Nb filaments to produce the ultimate superconducting wire product.
As aforesaid, the principal application of such superconducting materials is in magnets. In connection with this use, they must be wound upon a toroidal structure, preferably a D-shaped structure, as discussed in a paper by W. A. Fietz entitled "High Current Superconductors for Tokamak Toxoidal Field Coil" presented at the Applied Superconductivity Conference at Stanford, Calif., Aug. 17-20, 1976, and as discussed and illustrated in a Progress Report of May 1978 on "Force-Cooled Superconductor Development at the Francis Bitter National Magnet Laboratory at MIT." However, since the intermetallic compounds such as Nb.sub.3 Sn are very brittle, winding them onto a D-shaped coil whereby relatively sharp turns are imposed, presents some difficulty. One approach to this problem is to use a cable as flat as possible, generally referred to as a "Rutherford type" cable. However, the latter presents a further difficulty in that it has an insufficient void volume to permit adequate flow of the coolant. It is typically 97% dense with very few voids and cannot be cooled by forced flow of coolant through the conductor itself but only by a surrounding bath.
In more detail, a Rutherford cable is composed of two flat layers only. Each wire occupies a fixed position relative to all other wires in the cable such that no wire crosses over any other wire. The normal method of manufacturing can serve to describe the cable. The wires which form the cable are first formed into a series of spirals, generally on a mandrel. As the wires are slid off the mandrel they form first an oval then a flat rectangular cross-section. This is then further flattened to give some mechanical strength.
As commented on in the paper of W. A. Fietz, the conductor must be capable of withstanding mechanical stresses of three types without adverse effects on device performance. The first of these stresses occurs during the winding and handling of the conductor, presumably at room temperature. The conductor is subjected to bending, twisting tensile, and compressive forces between the time of its manufacture and the completion of winding of the device. A second critical stress period for the conductor may occur during cooldown of the device. Finally, the conductor must be designed to withstand the stresses of operating the energized device under any conceivable conditions, including quench. Although the stabilized conductor makes the possibility of quench remote, an event such as loss of coolant might result in a quench which the conductor must withstand without damage.
With regard to the function of the coolant, a paper by M. O. Hoenig et al on "Supercritical Helium Cooled, Cabled, Superconducting Hollow Conductors For Large High Field Magnets" presented at the 6th International Cryogenic Engineering Conference at Grenoble, France in 1976 stated that under steadystate conditions a DC current, carried by the superconducting filaments imbedded in the copper strands will generate no heating. Except to intercept external heating, the coolant is only needed to remove heat imposed by a transient instability. Since eddy currents can be virtually elminated by the transposition and twisting of conductor strands, the unscheduled instability will most likely come from wire motion. The imposition of such an instability results in the release of heat in the conductor. Due to its extremely low heat capacity the conductor temperature will rapidly rise, normalizing the superconductor and transferring current to its copper matrix. The system can be considered cryostable if the total current carried can be returned to the superconductor within a reasonable recovery period. Since the duration of the period is limited by the heat capacity of the coolant, cryostability becomes a function of coolant mass flow, permissible temperature rise and an adequate heat transfer coefficient.
Also of interest is a paper entitled "Stability Measurements of a Large Nb.sub.3 Sn Force-Cooled Conductor" (undated) by J. R. Miller et al of the Oak Ridge National Laboratory.
Thus objects of the invention are to attain high heat transfer from Nb.sub.3 Sn elements to the cooling fluid and a high inventory of cooling fluid within the conduit. Further objects are to provide mechanical support for the Nb.sub.3 Sn elements and protect them from damage due to mechanical stresses such as compaction, bending and the like.